Discussion on User Switching of LEO Satellite Mobile Communication System The low-orbit (LEO) satellite mobile communication system is a satellite communication system with a satellite distance of 500 to 1500 km from the ground and an operating period of 2 to 4 hours. Iridium system, global satellite system and system are the fastest-growing examples of earth-orbit satellite mobile communication systems. LEO satellite mobile communication system has broad development prospects The strongest signal criterion is that the terminal selects the satellite that can receive the strongest signal at any time. Having a sufficiently high signal strength is a basic condition of wireless communication, and it can be considered that the strongest signal satellite criterion can provide better quality of service.
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1 Characteristics of LEO satellite mobile communication system
Compared with the low-orbit (LEO) satellite mobile communication system and the medium-orbit (MEO) and geostationary (GEO) satellite mobile communication systems, it has the following characteristics:
1.1 Due to the smaller signal attenuation and lower propagation delay, the low-orbit satellite communication system is more conducive to personal global communications. The path transmission loss of the LEO system is usually several tens of decibels lower than that of GEO. The required transmission power is 1 / 200-1 / 2000 of GEO, and the propagation delay is only 1/7 ~ 1/50 of GEO. It is very advantageous to meet the delay requirements required for voice communication.
1.2 The development of cellular communication, multiple access, spot beam, frequency reuse and other technologies has provided technical guarantee for LEO satellite mobile communication.
1.3 Due to the large elevation angle of the ground mobile terminal to the satellite, the antenna beam is not easily affected by ground reflection, and multipath fading can be avoided.
1.4 It arranges multiple satellites on several orbital planes, and the satellites on multiple orbital planes are connected by inter-satellite communication links. The entire constellation is like a large platform that is structurally connected, forming a cellular service cell on the surface of the earth. Users in the service area are covered by at least one satellite, and users can access the system at any time.
1.5 Due to the high-speed movement of satellites and the large number of satellites, it also brings about problems such as serious Doppler frequency shift and complicated inter-satellite handover control. But in any case, the above-mentioned characteristics of the low-orbit satellite mobile communication system are very attractive to support the realization of personal communication.
2 General process of LEO satellite communication system user switching
In the low-orbit satellite mobile communication system, due to the high-speed movement of the satellite, its beam coverage area also moves, and the movement speed of the beam coverage area is much greater than the user's movement speed. Therefore, in the LEO satellite mobile communication system, the main switch It is caused by the movement of the satellite beam.
For the call handover in the satellite mobile communication system, it usually goes through such a process:
2.1 The user period measures the change in the signal strength of the pilot signal or broadcast channel of the currently used beam and the adjacent beam in order to determine whether it is crossing the boundary between adjacent beams or within the overlapping area of ​​adjacent beams.
2.2 If the user enters the overlapping area of ​​adjacent beams and meets the conditions for handover triggering, the handover process will start. The user stops using the current beam for communication and waits for the allocated channel to use the new beam for communication.
2.3 After the handover process starts, it is necessary to allocate channels for the user in the new arrival beam according to a certain channel allocation algorithm and release the used channels in the original beam; if intra-beam switching or channel rearrangement is used, the original beam The channel allocation within the beam must be optimized according to the channel rearrangement algorithm after the call ends, and the necessary intra-beam allocation must be performed. After the allocation is completed, the data flow is transferred from the old link to the new link to complete the handover.
3 Types of user switching in LEO satellite communication systems
Low-orbit satellite communication system user switching can be divided into the following types:
3.1 Switching between different beams of the same gateway and satellite
The target beam and the current beam are in the same gateway and the same satellite. The handover involves channel allocation of the two beams and modification of the routing table of the same gateway (not using on-board switching) or satellite (using on-board switching). .
3.2 Switching between different satellites at the same gateway
The target beam and the current beam are not in the same satellite, but within the range of the same gateway. It involves the channel allocation of two satellites. For the system using on-board switching, it is necessary to change the routing table of two satellites on-board switching For the satellite transparent forwarding system, it is necessary to modify the routing table of the gateway exchange.
3.3 Switching between beams of the same satellite at different gateway stations
The target beam and the current beam belong to the same satellite, but belong to different gateway stations. It involves the switching between the two gateway stations, including channel allocation, changing the ground line connection, location update, billing, etc. The satellite on the exchange also needs to change its exchange routing table.
3.4 Switching between different satellites at different gateway stations
The target beam and the first beam belong to different satellites and belong to different gateway stations. It involves the switching between two gateway stations and two satellites. The gateway station involves channel allocation, changing the ground line connection, location update, and memory. Fees and other issues, for satellites using on-board switching need to change their switching routing table.
4 Selection criteria for users to switch target satellites in LEO satellite communication system
In the handover control of the low-orbit satellite mobile communication system, the selection strategy of the target satellite for the handover also has a direct impact on the final performance of the handover. Therefore, according to the needs of the system, it is very important to design a choice of switching target satellites suitable for this system. At present, the selection strategies of handover target satellites in the low-orbit satellite mobile communication system mainly include the following: the nearest satellite criterion, the strongest signal criterion, the longest visible time criterion, the most available channel number criterion, the coverage time and elevation weighting criterion, and The minimum hop count switching criterion.
Among them, the recent satellite criterion considers that the satellite closest to the user terminal (the highest elevation angle) can provide a good quality of service (QoS), and its performance can be analyzed from a purely geometrical level, also known as the maximum elevation angle criterion. When using this criterion, the user terminal selects the satellite that can provide it with the highest elevation angle at any time. This criterion is simple to implement, but it is generally not used in actual systems because it does not consider the propagation conditions of wireless signals in the air, nor does it consider the operating conditions of the network.
The maximum visible time criterion is also called the maximum coverage time criterion. According to this strategy, users will use the prior knowledge of constellation system operation to always select the satellite with the largest service time as their target satellite for switching. This criterion is based on the consideration of the handover request arrival rate of the minimized system, which extends the time that the call has been served by a certain satellite after the handover, thereby obtaining a lower probability of forced interruption.
The criterion for the maximum number of available channels is: the user selects the satellite with the maximum number of available channels to provide services for it. This criterion is based on the consideration of channel resource utilization of the entire system, so that the traffic carried by each satellite in the satellite system tends to be evenly distributed to avoid failure due to overload of a certain satellite node, which affects the performance of the entire system. When applying this criterion, regardless of the specific location of the satellite, new calls and handover calls will experience the same blocking rate or the probability of being forced to be interrupted, thus avoiding the situation of a certain satellite being overloaded.
The minimum hop count switching criterion is applied to the case where there is an on-board route. The policy requires users to select the satellite that can provide the minimum hop count path at any time. In the specific implementation process, the two parties of the communication periodically check whether there are paths with fewer hops than the current communication path in their visible satellites, and switch if there is, otherwise they continue to use the current satellite for communication. Of course, if the current satellites on both sides of the communication are below the minimum elevation angle (or signal-to-noise ratio), they also need to be switched. Assuming that the satellite system uses a quasi-static routing algorithm, the routing table entry contains the satellite-to-satellite routing hops, and its routing information is automatically refreshed by the system as the network topology changes.
5 User switching and routing of low-orbit satellite communication systems
At the time of handover, due to changes in the service satellite, for satellite communication systems that use on-board switching and on-board routing, the original routing also needs to be re-established. There are several schemes for rebuilding routes: full route reconstruction, partial route reconstruction, rerouting combined with extended routing, dynamic probability optimization routing, and minimum hop routing.
Among them, the full route reconstruction satellite switching scheme: the original route is completely replaced by the new route. The new route obtained by this scheme is still the most optimized route, but its processing delay is relatively large.
Partial route reconstruction satellite switching scheme: When switching occurs, the original route is partially saved, and only the changed part is updated. The processing delay of this scheme is relatively small, but the newly generated route may not be the optimal route.
Combination of rerouting and extended routing: After switching, route expansion is performed first, and then route optimization is performed. To reduce the delay, but the signaling overhead increases.
Dynamic probabilistic optimized routing: Full route reconstruction saves bandwidth, but expands signaling resources, and it is necessary to select an appropriate optimized probability P to compromise between bandwidth and signaling resources. That is, not all the expanded routes are optimized, but a part of the routes are optimized with probability P, and some of them still maintain the original expanded routes.
Minimum hop routing strategy: The user selects the satellite that can provide the path with the least hops at any time. The two parties of the communication periodically check whether there are paths with fewer hops than the current communication path in their visible satellites, and switch if there is, otherwise they continue to use the current satellite for communication. This strategy can achieve lower propagation delay and smaller switching frequency, and has very good system performance.
references
[1] Chen Zhenguo, Yang Hongwen, Guo Wenbin. Satellite Communication System and Technology. Beijing: Beijing University of Posts and Telecommunications Press, 2003
[2] Liu Gang. Research on handover of low-orbit satellite constellation network. Journal of Communications, 2004 (25)
[3] Wang Liang, Zhang Naitong. Dynamic probability optimization strategy for satellite switching in LEO network. Journal of Communications, 2002